This invention relates to a process for the catalytic dewaxing of hydrocracked bright stock which improves bulk oxidation stability and storage stability. The term "oxidation stability" refers to the resistance of the oil to oxygen addition, in other words, how rapidly is oxygen picked up by and added to molecular species within the oil. Oxidation stability is indicated by the oxidator BN measured in hours. Oxidator BN is thoroughly described in U.S. Pat. No. 3,852,207 granted Dec. 3, 1974 to B. E. Stangeland et al at column 6, lines 15-30. Basically, the test measures the time required for 100 grams of oil to absorb one liter of oxygen. The term "storage stability" refers to the resistance of the oil to floc formation in the presence of oxygen.
In general, refineries do not manufacture a single lube base stock but rather process at least one distillate fraction and the vacuum residuum. For example, three distillate fractions differing in boiling range and the residuum may be refined. These four fractions have acquired various names in the refining art, the most volatile distillate fraction often being referred to as the "light neutral" fraction or oil. The other distillates are called "medium neutral" and "heavy neutral" oils. The residuum fraction is commonly referred to as "bright stock". Thus, the manufacture of lubricant base stocks involves a process for producing a slate of base stocks, which slate may include a bright stock.
Processes have been proposed to produce lubricating oil base stocks by refining bright stocks. Many of these processes use a hydrocracking step to produce a bright stock hydrocrackate which is in turn dewaxed to provide a dewaxed bright stock. The problem is that such hydrocracked stocks tend to have poor storage stability.
Moreover, since many process schemes proposed for hydrocracked stocks also involve the use of catalytic dewaxing to lower the pour point followed by solvent dewaxing to produce a dewaxed bright stock efforts have been made to develop improved dewaxing catalysts. Recently, it has been proposed to use shape-selective zeolitic dewaxing catalysts to crack, preferably hydrocrack, the paraffinic components contained in the bright stock.
Various zeolitic catalytic dewaxing processes have been proposed. For example: U.S. Pat. No. 4,472,266 (hydrodewaxing of lube oils with Mo, Ni-Mo or Co-Mo on ZSM-5 type catalysts); U.S. Pat. No. 4,437,976 (two-stage hydrocarbon dewaxing hydrotreating process using a hydrotreating catalyst from the class of ZSM-5, ZSM-11, ZSM-23, and ZSM-35 zeolites); and U.S. Pat. No. 4,222,855 (catalytic dewaxing of hydrocarbon oils using ZSM-23 or ZSM-35 zeolite catalysts).
A major problem is, however, that these catalytic dewaxing processes are adversely affected by catalyst poisons present in hydrocarbonaceous feedstocks. In particular, feedstocks often contain organic nitrogen that has detrimental effects upon zeolite catalytic activity. The narrow pore openings of the zeolites are quickly fouled and rendered ineffective by such catalyst poisons. In addition, the lubricating oil base stock derived from catalytic dewaxing of hydrocracked stocks is unstable in the presence of oxygen and light; further processing is required to make a stable oil.
In order to overcome the instability problems of hydrocracking stocks, the typical dewaxed hydrocrackate stock is hydrofinished by a mild hydrogenation process to increase the resistance of the bulk oil toward oxidation. The goal of this process is to hydrogenate those species which readily react with oxygen, while minimizing further cracking and loss of the lubricant base stock. Even though the hydrofinished product has high resistance toward bulk oxidation, its storage stability is often low. It is believed that this is due to the difficulty of totally saturating the floc-forming agents, thought to be partially hydrogenated polycyclic aromatics. These agents, upon reaction with oxygen, can lead to floc formation during storage of the oil.
There are several nonhydrogenation processing techniques recommended in the patent literature as methods to achieve improved lubricant storage stability. Some of the earlier efforts concentrated on the addition of stabilizing agents to a dewaxed hydrocrackate while in the presence of a heterogeneous acidic catalyst. Several issued patents relate to stabilizing hydrocracked lubricant base stocks by adding stabilizing agents such as olefins, alcohols, esters or alkylhalides to the lube stock while in the presence of a heterogenous acidic catalyst such as acid resins, clays, and alumino silicates having controlled alkylation activity. For instance, U.S. Pat. Nos. 3,928,171 and 4,181,597 disclose processes for stabilizing hydrocracked lube oils which have been dewaxed, preferably solvent dewaxed, by contacting them with stabilizing agents such as C.sub.6 to C.sub.10 olefins.
In spite of the large amount of research into developing lubricant base stocks, dewaxing and stabilizing them, the mechanism responsible for the benefits obtained when using a stabilizing agent was not entirely understood. Because the stabilizing agent is consumed during the stabilization reaction, however, it is likely that a reaction occurs between one or more components of the dewaxed lube oil stock and the stabilizing agent. In particular, conditions during the stabilization process are conducive to alkylation. Nonetheless, these earlier efforts refrain from asserting that any mechanism can be identified as the stabilizing reaction.
It has now been discovered that a three-step process comprising a first step to substantially remove nitrogen and sulfur contaminants, a second step to catalytically dewax, and a third step to thoroughly hydrogenate unstable polycyclics will produce a more stable dewaxed lubricating oil base stock from hydrocracked bright stock.